This invention relates to reduction of device damage in plasma processes, including DC (magnetron or non-magnetron) sputtering, and RF sputtering.
A typical plasma processing apparatus is shown in FIG. 1. The apparatus includes a plasma power supply 10, which drives a cathode or target 12 to a large DC voltage (e.g., -400 Volts) relative to the walls of vacuum chamber 14. The semiconductor substrate 16 (also known as the wafer) rests on a back plane 18 inside the chamber. The back plane may be driven by radio frequency (RF) AC voltage signals, produced by an RF power supply 20, which drives the back plane through a compensating network 22.
The AC and/or DC power supplies generate a plasma in the area above the wafer and between the wafer and the target, and cause material from the target to deposit on the wafer surface.
A typical DC power supply 10 includes a relatively sophisticated control system, designed to permit operation in constant power, constant voltage, or constant current modes. This control circuitry includes a damped control loop which, when the supply is engaged, produces a controlled ramping toward the desired output level. For example, as shown in FIG. 2, upon engagement of a typical DC power supply in an apparatus as shown in FIG. 1, the supply current (which represents the density of ionic transfer from the target due to sputter deposition on the wafer) ramps up to a constant value in a controlled manner with a small overshoot 24.
Despite the otherwise carefully regulated output produced by typical power supplies, it is normal to observe a spike in the target voltage during process initiation. As shown in FIG. 3, the magnitude of the spike 26 at process initiation may exceed the normal DC voltage level by a factor of 2 or more (e.g., those shown in FIG. 3 reach -1100 Volts). This phenomenon, known as the "break down" spike, is typically viewed as a necessary, isolated event associated with the creation of a plasma in the chamber 14 (otherwise known as "plasma ignition"). Furthermore, a large magnitude break down spike has been seen as necessary to improve process quality.